feat: 🎸 add limited support for multiplication

Author: alanhussey

README.md

@@ -362,3 +362,16 @@ eighteenInches.unit === inch;
 ```
 
 `subtract` does not support operating on more than 2 measurements at a time.
+
+#### `.multiply(measurement, ...numbers)`
+
+`multiply` will take the a measurement and multiply it with one or more numbers. The order of arguments is not important. Only one measurement can be supplied.
+
+```js
+const twelveInches = system.multiply(
+  m`4 inches`,
+  3
+);
+twelveInches.value === 12;
+twelveInches.unit === inch;
+```

src/UnitSystem/UnitSystem.js

@@ -1,4 +1,4 @@
-const { add, subtract } = require('../math');
+const { add, subtract, multiply } = require('../math');
 const Measurement = require('../Measurement');
 const Unit = require('../Unit');
 const Aliases = require('./Aliases');
@@ -102,6 +102,9 @@ class UnitSystem {
     const unit = measurements[0].unit;
     return subtract(...measurements.map(m => this.convert(m, unit)));
   }
+  multiply(...values) {
+    return multiply(...values);
+  }
 }
 
 module.exports = UnitSystem;

src/UnitSystem/UnitSystem.test.js

@@ -311,4 +311,15 @@ describe(UnitSystem, () => {
       ).toEqual(new Measurement(1, foot));
     });
   });
+
+  describe('#multiply', () => {
+    it('multiplies', () => {
+      const inch = new Unit('inch');
+      const system = new UnitSystem([[inch]]);
+
+      expect(system.multiply(new Measurement(3, inch), 2)).toEqual(
+        new Measurement(6, inch)
+      );
+    });
+  });
 });

src/createUnitSystem.js

@@ -18,10 +18,11 @@ function createUnitSystem(units) {
   const convert = bound(system, 'convert');
   const add = bound(system, 'add');
   const subtract = bound(system, 'subtract');
+  const multiply = bound(system, 'multiply');
 
   const m = createMeasurement(system);
 
-  return { m, createUnit, convert, add, subtract, system };
+  return { m, createUnit, convert, add, subtract, multiply, system };
 }
 
 module.exports = createUnitSystem;

src/createUnitSystem.test.js

@@ -11,6 +11,7 @@ describe(createUnitSystem, () => {
       convert: jasmine.any(Function),
       add: jasmine.any(Function),
       subtract: jasmine.any(Function),
+      multiply: jasmine.any(Function),
     });
   });
 

src/index.test.js

@@ -14,7 +14,14 @@ describe('createUnitSystem', () => {
   const { createUnitSystem, conversion } = index;
 
   test('creates a functional unit system', () => {
-    const { createUnit, m, convert, add, subtract } = createUnitSystem();
+    const {
+      createUnit,
+      m,
+      convert,
+      add,
+      subtract,
+      multiply,
+    } = createUnitSystem();
 
     const kilometer = createUnit('kilometer', {
       alias: 'km',
@@ -56,7 +63,7 @@ describe('createUnitSystem', () => {
         subtract(m`4 feet`, m`12 inches`),
         m`2 inches`,
         m(15.24, centimeter),
-        m`10 inches`,
+        multiply(m`5 inches`, 2),
         m`3 feet`
       )
     ).toEqual(m`7.5 feet`);

src/math/index.js

@@ -1,5 +1,7 @@
 const add = require('./add');
 const subtract = require('./subtract');
+const multiply = require('./multiply');
 
 exports.add = add;
 exports.subtract = subtract;
+exports.multiply = multiply;

src/math/multiply.js

@@ -0,0 +1,28 @@
+const Measurement = require('../Measurement');
+
+function _multiply(...numbers) {
+  return numbers.reduce((total, number) => total * number, 1);
+}
+
+function multiply(...values) {
+  const measurements = values.filter(value => value instanceof Measurement);
+  if (measurements.length === 0) {
+    throw new TypeError('Expected a Measurement but got none');
+  }
+  if (measurements.length > 1) {
+    throw new TypeError('Cannot multiply more than one Measurement');
+  }
+  const [measurement] = measurements;
+
+  const numbers = values.filter(value => !(value instanceof Measurement));
+  if (numbers.length === 0) {
+    throw new TypeError('Cannot multiply a Measurement with nothing');
+  }
+
+  return new Measurement(
+    _multiply(measurement.value, ...numbers),
+    measurement.unit
+  );
+}
+
+module.exports = multiply;

src/math/multiply.test.js

@@ -0,0 +1,41 @@
+const Measurement = require('../Measurement');
+const Unit = require('../Unit');
+const multiply = require('./multiply');
+
+describe(multiply, () => {
+  it('throws if a Measurement is not supplied', () => {
+    expect(() => multiply(2, 3)).toThrowError(
+      new TypeError('Expected a Measurement but got none')
+    );
+  });
+
+  it('throws if more than one Measurement is supplied', () => {
+    const foot = new Unit('foot');
+    expect(() =>
+      multiply(new Measurement(2, foot), new Measurement(3, foot))
+    ).toThrowError(new TypeError('Cannot multiply more than one Measurement'));
+  });
+
+  it('throws if just a single value is provided', () => {
+    const foot = new Unit('foot');
+    expect(() => multiply(new Measurement(2, foot))).toThrowError(
+      new TypeError('Cannot multiply a Measurement with nothing')
+    );
+  });
+
+  it('multiplies a Measurement and a number', () => {
+    const foot = new Unit('foot');
+    expect(multiply(new Measurement(2, foot), 3)).toEqual(
+      new Measurement(6, foot)
+    );
+    expect(multiply(5, new Measurement(4, foot))).toEqual(
+      new Measurement(20, foot)
+    );
+  });
+
+  it('multiplies a Measurement and multiple numbers', () => {
+    const foot = new Unit('foot');
+    const values = [4, 5, new Measurement(2, foot), 3];
+    expect(multiply(...values)).toEqual(new Measurement(120, foot));
+  });
+});